Italy joins the Allies in September 1939

What changes if Italy joins the Allies in early September 1939?

Britain can concentrate its fleet in northern waters, France gains southern support, and Germany faces pressure through Austria without an Axis Mediterranean threat.

Italian entry on the Allied side would threaten Germany through Austria and reinforce France, but the most immediate naval change is that Britain need not send Illustrious and other fleet units to the Mediterranean. Those forces become available for Norway and the North Atlantic. The scenario also exposes the risk of pausing British capital-ship work when the prewar fleet had received only limited maintenance and modernization.